This invention relates to a transponder, for example, for use in Registration and Identification Systems.
Transponders or antennae may be used in registration and identification systems, for example, automatic vehicle identification systems. The antennae (which are generally tuned antenna) are typically positioned such that they can transmit and receive signals over a specific area.
An automatic vehicle identification system (AVI) is one in which a transponder is mounted on a vehicle and is capable of generating a signal unique to that vehicle. The signal can be detected by any appropriate monitoring means. Generally this will be an antenna system, for example, that described in our co-pending U.S. patent Ser. No. 08/127,860 filed Sep. 28, 1993 and having a priority date of Sep. 28, 1992. The antenna will receive and interpret the signal from the vehicle mounted transponder and may use the signal, for example, for automatically charging motorway tolls, allowing entry into restricted areas, crime prevention, speed monitoring, etc.
Very often, it is necessary to mount the transponder directly onto a metal surface such as a car, container, etc. This has the fundamental problem, however, that the transponder can be de-tuned by the metal surface.
Previous solutions have been to detune the tank circuit of the antenna with a predetermined reverse shift. However, the mounting tolerances are then very critical. For ferro-metallic contact surfaces direct contact to the metal surface would still not be possible and thus the transponder may protrude too much away from the surface. This is obviously undesirable. Another solution is to correct the transponder frequency after the actual mounting using a separate ferrite rod. This has the disadvantage of high costs due to time consuming installation. In addition, read deterioration can occur due to low Q-factor when mounting the transponder on ferro metallic surfaces. A further disadvantage which occurs is lowered magnetic efficiency from the transponder antenna itself, due to "short-circuiting" of the field lines.
One object of the present invention is to provide a system which overcomes at least some of the disadvantages of present systems.